In 1519 Pope Leo X summoned Martin Luther to Rome, at this point Frederick the Wise, elector of Saxony (one of the major German princes) intervened and appealed to the Holy Roman Emperor Charles V. A compromise was reached whereby Luther and a lawyer appointed by the Pope, Thomas Cajetan, debated in Ausburg in Germany. The debate was inconclusive, neither side gave way and Luther was excommunicated the following year.
BUT it was a seminal moment in the formation of Protestantism as a political force. The debate took place on German soil with the Pope's representative forced to compromise in coming to Germany, instead of being able to summon a German subject at will, the decisions of Frederick and Charles to defend Luther signalled the end of Papal hegemony over the German states.
A little after this, following the diet or worms of 1521, and various changes in political alliances, the H.R Emperor turns tail and has Luther outlawed and declared a heretic - essentially a death sentence. But once again Frederick comes to the rescue and hides him at Wuttenberg castle, allowing him to write and to publicise his ideas.
There follows civil unrest, including protesting monks smashing up churches and general disorder throughout 'Germany' and the newly arrived printing press means that the Catholic authorities are unable to stem the tide of Protestant literature. What's more, the Turks are causing trouble to the east and so the attention of the Papal authorities are very much divided at this point, they soon realise it is a situation to be managed rather than conquered.
This leads to the first diet of Speyer in 1526, which temporarily undid the diet of Worms, removing the death sentence from Luther's head and un-censoring Protestant writings. Crucially during this diet the German Protestant Princes declared their faith for the first time and formed 'the league of Torgau' which included Phillip of Hesse and Frederick, which was later superseded by the 'Schmalkaldic League' in 1531, which had greater military and consequently political clout.
So, the answer to your question - probably by 1519, when Frederick of Saxony stood up to the Pope to protect Luther, but definitely, incontrovertibly, by 1526 when German Princes, who were heads-of-state and who had extra political clout through their role in election the Holy Roman Emperors, confessed themselves to be Protestants.
To put it very crudely, that was the 'oh shit' moment for the Catholic Church and note how quickly events progressed, from the Pope's summons in 1519 to the formation of the league of Torgau in 1526 - was only a span of 7 years. Suggesting that this was as much about deep seated political grievances as it was about theology. Luther struck lucky in appealing to a political class that was ready and waiting for a counter to Catholic doctrine which limited their own sovereignty.
The pace of change here is comparable to a modern revolution, it wasn't a slow, creeping thing, it was dramatically rapidly paced in the context of the period.
TLDR - Martin Luther had powerful political backers as some German princes embraced Protestantism as a means of standing up to the power of the Catholic Church.
The critics on the catholics church was as old as the bourgeoisie (~ 10th century). It was basically the same critic like Francis of Assisi did. Luther himself was from a bourgeoisie family, which made it possible for him to study first legal rights before he went into a monastery. At that time not only the bourgeoisie made critics on the church but the nobility too.
A factor for success was the new invented printing technology. The usage of propaganda writings became common and raised heavily in thefirst years when Luther became famous. Luther made a very good translation of the bible which became later the root of nowadays German because this bible translation was huge success and German wasn't existing at that time. So basically all of Luthers writings had a huge audience.
Luther himself made concepts of a church controlled by bishops and he wasn't successful with this, because the political powers of the kingdoms and cities took over control of religion. Each of the lords or city councils decided for the people whether they had to be catholic or protestantic and took over the leaderchip pof the churches. The was the next factor of the success of the reformation.
One of the factors for success was the constitution of Germany which at that time was a collection of kingdoms and independent cities like Hamburg with an emperor at the top. The power of the emperor was a composition of financing military with his own fortune and the good will of the lords of those kingdoms, which lead always to frictions. It were the lords and city councils who supported Luther.
The reformation can be interpreted as a result of the political frictions between emperor and the lords of the kingdoms which eventually started the 30 year war from 1618 to 1648. This war is usually interpreted as a war caused by religion, but it was the result of the political friction with religion as an excuse. Most of the protestantic powers came under heavy pressure until the swedish and french King saved them. Without the intervention of external powers this reformation would have been a failure. Sweden, Denmark and France tried to use the intervention to weaken the german emperor.
After 1648 the constitution was basically dead and the emperor was powerless and new kingdoms raised like prussia. Germany was divided into more than hundred principalities incl. borders with tariff walls.